Structural members, such as walls or columns, in buildings or bridges or other structural systems are often required to resist uplifting tensile forces and bending moments resulting from overturning actions caused by loads imposed on the structure due to its occupancy or external environmental actions, especially from the lateral loads of strong wind and earthquakes.
There is a large inventory of old structures in Canada and US and around the world which require repair or strengthening, rehabilitation or retrofit to restore or enhance their load carrying capacities to required performance level in order to ensure their safe use and operation. Enhancement of the tensile load or bending moment resistant capacities of individual structural members, and/or the restoration of deteriorated or damaged structural members to their pre-damaged capacities, are important parts of this process.
A practical means to enhance or restore the tensile load or bending moment capacity of a structural member is by adding external surfaced bonded reinforcing materials to the structural member. Thin steel plate or sheet has been used for this purpose. Recently since the 1990s, fiber reinforced plastic (FRP) sheets have been shown to be an attractive alternative to the steel plate. The FRP alternatives, which typically are of the types of carbon fiber reinforced plastic (FRP), glass fiber reinforced plastic (GFRP), aramid fiber reinforced plastic (AFRP), which is also commonly known by the trade name Kevlar, have the advantages of high strength, lightweight and excellent corrosion resistance compared to conventional reinforcing steel.
The conventional alternative FRP reinforcing system consists of bonding FRP sheets to the surface of the structural member by epoxy or other adhesives. The surface bonded FRP sheets provide additional tensile load resistance to the structural member in the direction parallel to its fiber direction. At the boundaries of the structural member to its supporting member or foundation, the load carried by the FRP sheets must be transferred to the supporting member or foundation. An anchorage system is critical for this load transfer and the effectiveness of the FRP strengthening system.
Previously, the anchorage system has an L-shaped angle anchor with one leg parallel to the FRP reinforced structural member and another leg parallel to the surface of the supporting element. The FRP sheet wrapping around the outer surfaces of the two legs of the angle is pressed against the surfaces of the structural member and the supporting element by the angle, which is in turn locked down to the supporting element by anchor bolts drilled through the surface of one leg of the angle (see FIG. 1). Because of the eccentricity between the loading direction of the FRP sheet and the hold down of the angle to the supporting member, there is significant bending or prying action to the angle shape resulting in large out-of-plane distortion of the FRP sheet from its loading plane. This leads to a reduced load carrying capacity and resistance by the FRP sheet, especially under cyclic load applications when the FRP sheet is repeatedly subjected to loading and unloading causing break or cut to the fiber due to the repeated cycles of out-of-plane deformations and warping. The premature failure of the FRP reinforcing system is due to the eccentricity between the load carried by the FRP sheet and the lock-down resistance from the angle anchorage system.
Another challenge when using FRP for structural reinforcement is the problem of debonding of the FRP sheet from the supporting member or foundation. Nanni et at. (A. Nanni, Khalifa, A., T. Alkhrdaji and S. Lansbury, “Anchorage of Surface Mounted FRP Reinforcement”, Concrete International: Design and Construction, Vol. 21, No. 10, October 1999, pp. 49–54) attempted to employ a U-shaped anchor to prevent such debonding in beams reinforced with FRP sheets. In Nanni et al., a U-anchor is embedded at a bent portion of the end of the FRP reinforcement sheet into a preformed groove in the supporting member or foundation (see FIG. 2). The goal is to develop anchorage force in the U-anchor by embedment of the FRP sheet. Viscous paste is used to fill the groove. Optionally, the end portion of the FRP sheet may wrap around a FRP bar inside the groove. However, it is apparent that the FRP bar has no bearing on the exertion of anchorage force. Furthermore, the viscous paste may not be strong enough to hold the FRP sheet inside the groove.
Furthermore, in Nanni et al., the working principle of the U-anchor system is that the load transfer from the FRP sheet to the concrete base is highly dependent on the shear and tensile strength of the bond between the FRP sheet and the concrete on the inside surface of the groove. The U-anchor arrangement is just a means to increase the length of this bond area available for the transfer of the load, eccentricity still exists between the tensile force carried by the FRP sheet on the vertical web of the beam and the resultant anchor resistance provided by the bond between the FRP sheet and the concrete distributed over the circular inside surface of the groove.
Accordingly, there is a need for an improved anchoring system whereby the system is able to provide an inherent concentric centering capability in the load transfer mechanism and to eliminate the undesirable prying action effect. The present invention is for a new self-centering anchorage system which eliminates the eccentricity problem and allows the FRP material to fully utilize its high strength without premature failure.